1. Field of the Invention
The invention relates to exhaust emission control method and system for treating exhaust gas emitted from an internal combustion engine.
2. Description of Related Art
In a direct injection type internal combustion engine, such as a diesel engine, installed on an automobile, or the like, it is generally required to remove nitrogen oxides (NOx) contained in exhaust gas. To meet this requirement, it has been proposed to provide a NOx storage agent as one type of exhaust emission control means in an exhaust gas passage of the internal combustion engine.
The NOx storage agent used for removing NOx has the function of storing NOx when the air/fuel ratio of the exhaust gas is lean, and releasing the stored NOx for reduction and removal when the air/fuel ratio of the exhaust gas is reduced and a reductant, such as HC and CO, is present in the exhaust gas. By utilizing this function, the NOx storage agent stores NOx in the exhaust gas when the air/fuel ratio of the exhaust gas is lean, and reduction and removal of the NOx stored in the NOx storage agent are effected by, for example, supplying a reductant (e.g., fuel) to the NOx storage agent when or before the storage capacity of the NOx storage agent is reduced after a certain period of use.
The “storage” used in the present specification includes the meaning of both “absorption” and “adsorption”. Here, “absorption” refers to storage of NOx in the form of nitrates, or the like, and “adsorption” refers to adsorption of NOx in the form of NO2, or the like.
In the meantime, fuel used in the internal combustion engine may contain a sulfur component (S), and, in this case, exhaust gas emitted as a result of combustion of the fuel contains sulfur oxides (SOx). If SOx is present in the exhaust gas, the NOx storage agent stores SOx in the exhaust gas through the same mechanism as that for storing NOx.
It is, however, to be noted that SOx stored by the NOx storage agent is relatively stable, and generally tends to be accumulated in the NOx storage agent. If the amount of SOx accumulated in the NOx storage agent is increased, the NOx storage capacity of the NOx storage agent is reduced, and the NOx storage agent will not be able to sufficiently remove NOx from the exhaust gas. Thus, a problem of so-called sulfur poisoning (or S poisoning) occurs which results in a reduction of the NOx conversion efficiency. Particularly, the problem of sulfur poisoning is likely to occur in a diesel engine that uses light oil having a relatively large content of sulfur component as a fuel.
It is also known that the NOx storage agent is able to release SOx stored in the NOx storage agent through the same mechanism as that for releasing NOx. However, since SOx is stored in the NOx storage agent in a relatively stable form, it is difficult for the NOx storage agent to release SOx stored therein at a temperature (e.g., about 250° C. or higher) at which NOx is normally reduced and removed. In order to eliminate sulfur poisoning, therefore, sulfur regeneration control needs to be performed periodically, in which the temperature of the NOx. storage agent is raised to a sulfur release (or desulfurization) temperature (e.g., 600° C.) or higher, which is higher than the NOx reduction/removal temperature, and the air/fuel ratio of the exhaust gas flowing through the NOx storage agent is controlled to be substantially equal to the stoichiometric air/fuel ratio or rich (hereinafter simply said “controlled to be rich”). The desulfurization temperature mentioned herein is the lowest temperature that permits sulfur to be released from the NOx storage agent.
One method of performing the sulfur regeneration control as disclosed in, for example, Japanese Laid-open Patent Publication No. 2000-291462, utilizes so-called low-temperature combustion. The low-temperature combustion is performed by recirculating a considerably large amount of exhaust gas from the exhaust side to the intake side of the engine, and causing combustion in the combustion chamber while the temperature of fuel and gas surrounding the fuel is kept at a relatively low level by utilizing the endothermic effect of the recirculated gas (EGR gas), so as to suppress generation of smoke. If the low-temperature combustion is performed in this manner, a large amount of a reductant (such as HC and CO) is contained in the exhaust gas, and the temperature of the NOx storage agent disposed in the exhaust gas passage is raised to the desulfurization temperature or higher mainly due to the reaction of the reductant. In this case, since differences in the temperature among respective portions of the NOx storage agent are relatively small, almost the entire volume of the NOx storage agent can be regenerated or recovered from sulfur poisoning by effecting low-temperature combustion while the air/fuel ratio is kept rich, or injecting a reductant at a location upstream of the NOx storage agent thereby to make the air/fuel ratio of the exhaust gas rich.
However, if the required torque of the engine is increased, namely, if a fuel injection quantity is increased, it becomes difficult to perform the low-temperature combustion because the temperature of fuel and surrounding gas is raised at the time of combustion. Namely, the low-temperature combustion can be performed only when the engine is in a low-load operating state in which the quantity of heat generated due to combustion is relatively small. Thus, the sulfur regeneration method for regenerating the NOx storage agent by utilizing the low-temperature combustion cannot be performed while the engine is operating at a high load.
In another known method of performing the sulfur regeneration control as disclosed in, for example, Japanese Laid-open Patent Publication No. 11-350949, fuel serving as a reductant is injected into an exhaust gas passage upstream of the NOx storage agent. In this case, the temperature of the NOx storage agent is raised due to reaction of the fuel thus injected. This method enables sulfur regeneration to be carried out even in an engine operating region in which the above-described low-temperature combustion cannot be performed.
However, if the injection of the fuel or reductant is employed for keeping the air/fuel ratio in a rich region, the temperature of the NOx storage agent is gradually increased due to reaction of the fuel or reductant in the NOx storage agent, and may exceed a heat deterioration temperature at which the NOx storage agent suffers heat deterioration, depending upon the operating state of the engine. In particular, when the fuel or reductant injected into the exhaust gas passage has a relatively low reactivity, the temperature is normally likely to be raised in a downstream portion of the NO storage agent, and this portion may be excessively heated, namely, its temperature may exceed the heat deterioration temperature.